In oil field production operations, produced fluids are oil and water fluid mixtures that require specialized separation processing. It is desirable to separate the oil from the water so that the oil can be sold and the water can be re-used (e.g., re-injected back into the reservoir to maintain reservoir pressures). It is well known that the water recycled from the production fluid separation processes must be of sufficient quality for re-injection, particularly where the water is mixed with polymers or other chemicals prior to the re-injection.
To date, however, existing separation processes prove ineffective because the produced oil-water mixture is often an emulsion that can be difficult to separate, particularly where the emulsion contains additional production-enhancing chemicals. Known processes also prove troublesome where the produced oil-water mixtures contain sand or other debris (e.g., for production from fracked wells, it is common for 10-15 cubic meters/month of sand to accumulate during separation processes when producing 3000-5000 BPD fluid).
Skim tanks are very large gravity separation vessels that can be used to separate the oil from the water. Oil, having a lower density than water, rises to the surface of the water. During separation, oil droplets coalesce together and float to the top of the surface of the water, where it can be removed. Many attempts have been made over the past half century to improve the efficiency of skim tanks including, for example, configuring tanks to increase the tank's residence or retention time, allowing more time for the oil to rise through the water. Empirical retention time, as opposed to theoretical retention time, is commonly defined as the total amount of time that a molecule of fluid remains in the tank before exiting the tank. Other attempts to improve the efficiency of skim tanks have included modifying the tank to include different piping designs such as horns, nozzles, deflectors and/or accumulators to assist with the separation.
More specifically, some existing tanks have been designed with the aim of providing a longer flow path for the production fluids to travel, such flow path often incorporating coalescing structures to enhance separation. For example, as described in U.S. Pat. No. 4,555,332, tanks have been configured to form a continuous fluid flow channel through coaxially arranged cylindrical chambers formed by baffles within the tank. U.S. Pat. No. 4,844,819 discloses a vertical separation vessel specifically configured to enhance the overall (plastic) surface area of the tank, such that oil, which has an affinity to the plastic, adheres thereto and coalesces into larger globules.
Other known tanks are designed to incorporate modified inlet piping (e.g., conical-shaped inlet diffusers) that cause the fluid flow to achieve greater horizontal width when introduced to the tank, while minimizing vertical divergence (and thus turbulent mixing) of the fluid as it enters the tank.
To date, however, known skim tanks continue to fall short in providing greater than approximately 50% of theoretical retention time at best, which is a result of water short-circuiting to the outlet nozzles and decreasing tank efficacy. As a result, many operators are forced to take additional measures to remove residual oil in the water, e.g., installing filters on the outlet stream, or adding additional tanks to increase overall retention time, such measures increasing the overall costs of the separation process.
There remains a need for an improved system that proves effective at removing residual oil from production water resulting from primary separation processes following oil well production operations, particularly where the resulting production water contains approximately 99% water. It is desirable that such an improved system alters the specific gravity and increases the particle size of the oil phase within the production water, and also provides a sufficient actual (empirical) retention time (e.g., 50% and up to 80% of theoretical retention time) so as to efficiently and effectively perform the separation process.